Porous ceramic and method for production thereof

ABSTRACT

The method for producing a porous ceramic comprises the slurry preparing step of preparing a slurry containing a powdery aggregate and a hydrated crystalline material having a burn-out feature, the solid matter forming step of decreasing a liquid component from the slurry prepared in the slurry preparing step, thereby forming a solid matter, and the firing step of firing the solid matter formed in the solid matter forming step to burn up the hydrated crystalline material, thereby forming the porous ceramic.

TECHNICAL FIELD

The present invention,

The present invention relates to a porous ceramic and a method forproducing the same.

BACKGROUND ART

About methods for producing a porous ceramic, various contrivances havebeen made so far.

For example, there is a method for producing a porous ceramic, whereinthe diameter of raw material powder is adjusted, thereby generating, inthe grain boundary between crystal grains, pores which do not disappearat the time of sintering.

There is also a method for producing a porous ceramic, wherein sinteringtemperature is made lower than a given temperature when compared withthe prior art, thereby causing pores which disappear at conventionalsintering temperatures to remain.

Furthermore, there have also been developed a method of preparing aslurry containing ceramic powder and having foaming property, turning itinto a whip-form slurry, and molding and firing the slurry; a method offilling a mold with a synthetic resin foamed body, filling pores thereinwith a slurry containing ceramic powder and having self-hardeningability, heating and hardening the resultant, and firing the obtainedmolded body; and a method of mixing combustible particles made ofcarbon, coffee husks or the like with ceramic powder and then firing amolded body therefrom so as to burn up the combustible particles.

In general, however, in the production of a ceramic product, forexample, powder having a wide particle diameter distribution, whereinsilica rock, feldspar or the like is finely pulverized, is used asstarting powder. As clay, a material which has already been fine fromthe production time thereof and further has a wide particle diameterdistribution is used. That is, the scattering in particle diameters ofstarting materials is very large. It is therefore difficult in methodsfor producing a porous ceramic in the prior art as described above thatthe porosity, the pore size, the strength and others of the resultantporous ceramic are minutely adjusted.

Specifically, when an attempt for producing a porous ceramic havingdesired porosity and pore size is made by the method in which thediameter of staring powder is adjusted, a labor for adjusting theparticle diameter of the starting powder is required and furtherstarting powder waste is generated.

When an attempt for producing a porous ceramic having desired porosityand pore size is made by the method in which sintering is conducted at alower temperature than a given temperature, a high-strength porousceramic cannot be produced since sufficient sintering is not attained.

It is also difficult to minutely adjust the volume of spaces distributedbetween starting powders by the method using a whip-form slurry, asynthetic resin foamed body or combustible particles.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a method for producinga porous ceramic having a small scattering in pore sizes.

A first method for producing a porous ceramic of the present inventioncomprises:

the slurry preparing step of preparing a slurry containing a powderyaggregate and a hydrated crystalline material having a burn-out feature,

the solid matter forming step of decreasing a liquid component from theslurry prepared in the slurry preparing step, thereby forming a solidmatter, and

the firing step of firing the solid matter formed in the solid matterforming step to burn up the hydrated crystalline material, therebyforming the porous ceramic.

Herein, the hydrated crystalline material means a material which canform a hydrated crystal correspondingly to the temperature, the pressureor some other state of the surrounding atmosphere thereof, or a changeembodiment thereof when the material is dispersed in a solvent whichcontains water.

In the first method for producing the porous ceramic of the presentinvention, the hydrated crystalline material may be trehalose.

In the first method for producing the porous ceramic of the presentinvention in this case, a given thermal hysteresis may be given to theslurry so as to set the size of crystal of the trehalose to a given sizein the slurry preparing step.

In the first method for producing the porous ceramic of the presentinvention, in the slurry preparing step a mucopolysaccharide may beincorporated in the slurry.

In the first method for producing the porous ceramic of the presentinvention in this case, the mucopolysaccharide may be hyaluronic acid

In the first method for producing the porous ceramic of the presentinvention, in the slurry preparing step a binder for binding powders ofthe powdery aggregate to each other is incorporated in the slurry.

In the first method for producing the porous ceramic of the presentinvention in this case, the binder may be at least one selected fromsodium silicate, colloidal silica, alumina sol, lithium silicate, glassfrit, natural rubber, synthetic rubber, methylcellulose,carboxymethylcellulose, hydroxymethylcellulose, hydroxyethylcellulose,polyvinyl alcohol, polyethylene, phenol resin, polyacrylate, and sodiumpolyacrylate.

In the first method for producing the porous ceramic of the presentinvention, in the slurry preparing step at least one selected from thefollowing may be incorporated in the slurry: a fluidizing agent, adispersing agent, a slurry hardening agent, an organic monomer and acrosslinking agent for the monomer, a plasticizer, an antifoamer, and asurfactant.

In the first method for producing the porous ceramic of the presentinvention, in the slurry preparing step a burn-out feature materialwhich is different from the hydrated crystalline material may beincorporated in the slurry.

In the first method for producing the porous ceramic of the presentinvention, in the slurry preparing step the slurry may be cooled fromthe outside.

In the first method for producing the porous ceramic of the presentinvention, in the solid matter forming step the humidity of thesurrounding atmosphere of the slurry may be lowered.

In the first method for producing the porous ceramic of the presentinvention, in the solid matter forming step the pressure of thesurrounding atmosphere of the slurry may be lowered.

In the first method for producing the porous ceramic of the presentinvention, in the solid matter forming step the slurry may bespray-dried, thereby forming the solid matter in a powder or granularform.

In the first method for producing the porous ceramic of the presentinvention, in the solid matter forming step a filter may be used todecrease the liquid component of the slurry, thereby forming the solidmatter.

In the first method for producing the porous ceramic of the presentinvention, in the firing step the solid matter may be oxidization-fired.

In the first method for producing the porous ceramic of the presentinvention, in the firing step the solid matter may be reduction-fired.

In the first method for producing the porous ceramic of the presentinvention, in the firing step the pressure of the surrounding atmosphereof the solid matter may be changed while the temperature thereof is keptconstant.

In the first method for producing the porous ceramic of the presentinvention, in the firing step the temperature of the surroundingatmosphere of the solid matter may be changed while the pressure thereofis kept constant.

According to the first porous ceramic producing method of the presentinvention, there can be obtained a porous ceramic produced by firing asolid matter formed by decreasing a liquid component from a slurrycontaining a powdery aggregate and a hydrated crystalline materialhaving a burn-out feature so as to burn up the burn-out featurematerial.

In the above-mentioned porous ceramic, the hydrated crystalline materialmay be trehalose.

The above-mentioned porous ceramic may carry at least one selected froman electrifiable material, a material having an ion exchange group, amaterial having a hydrophilic functional group, a material having ahydrophobic functional group, a catalyst, an enzyme, a protein, asaccharide, a lipid, and a nucleic acid; a modified product of any onethereof, a composite of plural materials selected therefrom; and otherceramic materials, and glass.

In the above-mentioned porous ceramic, a surface-covering layer may beformed.

In the above-mentioned porous ceramic, the surface thereof may besubjected to chemical and/or physical surface treatment.

In the above-mentioned porous ceramic, the pore density of an outerportion may be higher than that of an inner portion.

A second method for producing a porous ceramic of the present inventioncomprises:

the slurry preparing step of preparing a slurry containing a powderyaggregate, a burn-out feature material, and a mucopolysaccharide,

the solid matter forming step of decreasing a liquid component from theslurry prepared in the slurry preparing step, thereby forming a solidmatter, and

the firing step of firing the solid matter formed in the solid matterforming step to burn up the burn-out feature material, thereby formingthe porous ceramic.

In the second method for producing the porous ceramic of the presentinvention, the mucopolysaccharide may be hyaluronic acid.

In the second method for producing the porous ceramic of the presentinvention, in the slurry preparing step aggrecan may be incorporated inthe slurry.

In the second method for producing the porous ceramic of the presentinvention, in the slurry preparing step a binder for binding powders ofthe powdery aggregate to each other may be incorporated in the slurry.

In the second method for producing the porous ceramic of the presentinvention, the binder may be at least one selected from sodium silicate,colloidal silica, alumina sol, lithium silicate, glass frit, naturalrubber, synthetic rubber, methylcellulose, carboxymethylcellulose,hydroxymethylcellulose, hydroxyethylcellulose, polyvinyl alcohol,polyethylene, phenol resin, polyacrylate, and sodium polyacrylate.

In the second method for producing the porous ceramic of the presentinvention, in the slurry preparing step at least one selected from thefollowing may be incorporated in the slurry: a fluidizing agent, adispersing agent, a slurry hardening agent, an organic monomer and acrosslinking agent for the monomer, a plasticizer, an antifoamer, and asurfactant.

In the second method for producing the porous ceramic of the presentinvention, in the solid matter forming step the humidity of thesurrounding atmosphere of the slurry may be lowered.

In the second method for producing the porous ceramic of the presentinvention, in the solid matter forming step the pressure of thesurrounding atmosphere of the slurry may be lowered.

In the second method for producing the porous ceramic of the presentinvention, in the solid matter forming step the slurry may bespray-dried, thereby forming the solid matter in a powder or granularform.

In the second method for producing the porous ceramic of the presentinvention, in the solid matter forming step a filter may be used todecrease the liquid component of the slurry, thereby forming the solidmatter.

In the second method for producing the porous ceramic of the presentinvention, in the firing step the solid matter may be oxidization-fired.

In the second method for producing the porous ceramic of the presentinvention, in the firing step the solid matter may be reduction-fired.

In the second method for producing the porous ceramic of the presentinvention, in the firing step the pressure of the surrounding atmosphereof the solid matter may be changed while the temperature thereof is keptconstant.

In the second method for producing the porous ceramic of the presentinvention, in the firing step the temperature of the surroundingatmosphere of the solid matter may be changed while the pressure thereofis kept constant.

According to the second porous ceramic producing method of the presentinvention, there can be obtained a porous ceramic produced by firing asolid matter formed by decreasing a liquid component from a slurrycontaining a powdery aggregate, a burn-out feature material, and amucopolysaccharide so as to burn up the burn-out feature material.

In the above-mentioned porous ceramic, the mucopolysaccharide may behyaluronic acid.

The above-mentioned porous ceramic may carry at least one selected froman electrifiable material, a material having an ion exchange group, amaterial having a hydrophilic functional group, a material having ahydrophobic functional group, a catalyst, an enzyme, a protein, asaccharide, a lipid, and a nucleic acid; a modified product of any onethereof, a composite of plural materials selected therefrom; and otherceramic materials, and glass.

In the above-mentioned porous ceramic, a surface-covering layer may beformed.

In the above-mentioned porous ceramic, the surface thereof may besubjected to chemical and/or physical surface treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the structural formula of trehalose.

FIG. 2 is a view showing the structural formula of hyaluronic acid.

FIG. 3 is a view showing a sectional form of a porous ceramic.

BEST MODES FOR CARRYING OUT THE INVENTION

Methods for producing a porous ceramic according to embodiments of thepresent invention are described hereinafter.

Embodiment 1

In the method for producing a porous ceramic according to Embodiment 1of the present invention, first, in a slurry preparing step, a slurrycontaining a powder aggregate and a hydrated crystalline material havinga burn-out feature is prepared.

Next, in a solid matter forming step, a liquid component is decreasedfrom the slurry prepared in the slurry preparing step, thereby forming asolid matter. The solid matter obtained at this time is a matter whereinthe hydrated crystalline material is dispersed in the matrix of thepowder aggregate.

In a firing step, the solid matter is fired. At this time, the hydratedcrystalline material is burned up to form a porous ceramic.

The powdery aggregate constitutes the skeleton of the porous ceramic.Examples of the powdery aggregate include pottery, porcelain material,alumina, forsterite, cordierite, zeolite, hydrotalcite,montomorillonite, sepiolite, zirconia, silicon nitride, silicon carbide,calcium phosphate, hydroxyapatite. Additionally, glass fiber, rock wool,almunosilicate fiber and alumina fiber, which are heat-resistant fibers,are exemplified. The means for adjusting the particle diameter of thepowdery aggregate may be a method using a ball mill. Examples of theshaping method for it include mold-press, cold isostatic press, castmolding, injection molding, extrusion molding, doctor blade methods, andthe like.

The hydrated crystalline material has burning-up property, and isdispersed and contained in the solid matter obtained by removing watercontent from the slurry and is burned up by firing, so as to form poresin the porous ceramic. The hydrated crystalline material has a verysmall scattering in sizes of particles thereof when the material isincorporated in the slurry. For this reason, the resultant porousceramic has a very small scattering in pore sizes. The hydratedcrystalline material is, for example, trehalose.

FIG. 1 is a view showing the structural formula of trehalose.

Trehalose is a non-reducing disaccharide having a molecular weight of342.30 wherein two molecules of D-glucose are 1,1-bonded to each other.When trehalose is dissolved in water, it turns into an aqueous solutionin a supersaturated state by a rise in the temperature of the water. Bya fall in the temperature, it turns into hydrated crystal originatingfrom crystal nuclei. In addition, the trehalose hydrated crystal isdissolved at 80 to 90° C. At 100° C. or higher, water content isevaporated and scattered therefrom so that anhydration starts. Trehaloseconverted to anhydrous crystal with the rise in the temperature thereofcarbonizes at about 600° C. When the temperature is further raised up inan oxidization atmosphere, trehalose is burned at about 800 to 900° C.to turn to CO₂. On the other hand, the powdery aggregate has not yetbeen sintered in this step, and the fluidity of spaces between powdersof the powdery aggregate is kept. Therefore, the porous structure of thepores ceramic can be adjusted into various forms. Trehalose is containedin mushrooms in many cases. The reason why a dried shiitake mushroom iscompletely restored into an original form when it is immersed into wateris that trehalose contained in the shiitake mushroom forms hydratedcrystal or trehalose attracts surrounding water molecules intensely soas to turn into a stable state. The fact that confectionery has afunction of keeping freshness is also because the confectionery containstrehalose.

When trehalose is incorporated in the slurry, trehalose is dissolved upto a supersaturated state wherein the ratio of the weight of trehaloseto that of hot water (W/W) is three or more. With a fall in thetemperature thereof, crystal is formed from starting points of crystalnuclei in the slurry. As the trehalose content in the slurry is larger,the mass of bi-hydrated crystal of trehalose which enters the solidmatter at the time of removing water content in the solid matter formingstep is larger. Accordingly, the porosity of the resultant porousceramic can be adjusted by controlling the amount of dissolvedtrehalose.

When the temperature of the slurry is gently and gradually lowered,large crystals of trehalose are formed. On the other hand, when thetemperature of the slurry is rapidly lowered, a great number of smallcrystals are formed. Therefore, a given therma hysteresis is given tothe slurry, whereby the crystal growth of trehalose is adjusted to varythe diameter of the crystals. As a result, the pore size of theresultant pore ceramic can be adjusted.

As described above, when trehalose is used as the hydrated crystallinematerial, hydrated trehalose crystals which are homogenous and havedesired sizes can be realized in the slurry. Consequently, a porousceramic having a pore structure corresponding to it can be obtained.

A mucopolysaccharide may be incorporated in the slurry. Amucopolysaccharide is the generic name of any polysaccharide whichcontains an amino sugar and uronic acid. A mucopolysaccharide absorbswater content in the slurry to remove the water content, and promotescrystallization of soluble components dissolved in the slurry. Examplesof the mucopolysaccharide include hyaluronic acid, chondroitin sulfate,heparin, keratan sulfate, and the like. Among these, hyaluronic acid,which is excellent in water absorbing performance, is preferable.

FIG. 2 is a view showing the structural formula of hyaluronic acid.

Hyaluronic acid is a macromolecular mucopolysaccharide having, as aminimum recurring unit, a disaccharide of glucuronic acid andN-acetyl-D-glucosamine. Hyaluronic acid has a property of attracting agreat amount of water and attracts water content in a volume 6000 timesthe weight thereof. Therefore, only by incorporating a very small volumeof hyaluronic acid in the slurry, hyaluronic acid absorbs most all ofwater content in the slurry in a liquid form, so that the slurry isstabilized. That is, by the fixation of excessive water content, thefluidity of the slurry which is close to the critical point of sol-gelis reduced at a stretch to result in gelatinization.

A binder for bonding powders of the powdery aggregate to each other maybe incorporated in the slurry. In the case that the volume of theburn-out feature component distributed between the powdery aggregatepowders is large, the bonds between the powdery aggregate powders areliable to become brittle. However, the binder causes pore spaces betweenthe powdery aggregate powders to be kept while the binder makes strongmutual bonds between them. Examples of the binder include sodiumsilicate, colloidal silica, alumina sol, lithium silicate, glass frit,gua gum (natural rubber, or synthetic rubber), methylcellulose,carboxymethylcellulose, hydroxymethylcellulose, hydroxyethylcellulose,polyvinyl alcohol, polyethylene, phenol resin, polyacrylate, and sodiumpolyacrylate. Any one of these or two or more thereof may beincorporated as the binder(s) in slurry.

At least one selected from the following may be incorporated in theslurry: a fluidizing agent such as sodium silicate, a dispersing agentsuch as polycarboxylic acid, a slurry hardening agent of amine type orthe like, an organic monomer and a crosslinking agent for the monomer, aplasticizer, an antifoamer, and a surfactant such as sugar ester orpolygly ester. When such an additive is incorporated in the slurry,properties of the slurry or the solid matter change. Properties of theresultant porous ceramic, such as the strength thereof and the formingmanner of the pores, are affected and changed accordingly. For example,when a surfactant is incorporated in the slurry, the state that water isblended with the powdery aggregate is made more homogeneous so that thehomogeneity of the resultant pore ceramic becomes high. Consequently, byincorporating a given additive in the slurry dependently on purpose, adesired porous ceramic can be obtained.

A burn-out feature material which is different from the hydratedcrystalline material may be incorporated in the slurry. By incorporatingthe burn-out feature material which is different form the hydratedcrystalline material in the slurry, a porous ceramic wherein pluralkinds pores having pore sizes different from each other are intermixedcan be obtained. For example, in a porous ceramic obtained from a slurryprepared by incorporating coffee husks into a powdery aggregate made ofcordierite in an amount of 10 mass % thereof and then dissolving theresultant into a aqueous saturated trehalose solution at the temperatureof which is adjusted to 40° C., small pores based on trehalose and largepores based on the coffee husks are intermixed. The porous ceramichaving the pores with the large and small pore sizes is suitable for afilter having high permeability and a bone scaffold having goodbiocompatibility. Examples of the burn-out feature material which isdifferent from the hydrated crystalline material include graphite, wheatflour, starch, phenol resin, polymethyl methacrylate resin, polyethyleneresin, polyethylene terephthalate resin, and the like. Any one of theseor two or more thereof may be incorporated as the burn-out featurematerial(s) in the slurry.

Besides, an appropriate amount of one or more of the following organiccompounds may be incorporated in the slurry: other sugars and aminoacids, such as glucose, maltose, isomerized sugars, sucrose, honey,maple sugar, palatinose, sorbitol, xylitol, lactitol, maltitol,dihydrochalcone, stevioside, α-glycosylstevioside, rakanka sweet tastematerial, glycyrrhizin, L-aspartyl-L-phenylalanine methyl ester,sucralose, Acesulfame K, saccharin, glycine, alanine, dextrin, starch,and lactose.

In the slurry preparing step, the slurry may be cooled from the outside.In this case, the temperature of the slurry falls from the outside tothe inside successively, and in the outer portion crystal of thehydrated crystalline material is formed in a larger amount from theaggregate particles as nuclei. Accordingly, in the porous ceramicobtained by firing this as a solid matter, the pore density thereof ishigher in a portion closer to the surface, that is, in an outer portionrather than in the inside thereof.

In the solid matter forming step, water content is removed from theslurry by such means as hot wind drying, freeze-drying, super criticaldrying, or spray drying. By lowering the humidity of the surroundingatmosphere of the slurry, the evaporation of the water content from theslurry may be promoted. Furthermore, by lowering the pressure of thesurrounding atmosphere of the slurry, the evaporation of the watercontent from the slurry may be promoted.

In the solid matter forming step, a powdery or granular solid matter maybe formed by spray-drying the slurry in a spray drying machine (spraydryer). Specifically, the slurry is blown out from nozzles to a dryingchamber in a heated state, so as to prepare liquid droplets in a fineparticle form, and the liquid droplets are brought into contact with hotwind and dried, thereby yielding a powdery or granular solid matter. Thespray drying machine is classified into a disc atomizer, a two-fluidnozzle, a four-fluid nozzle, or other types, dependently on differencein its pulverizing device. An appropriate type is selected in accordancewith physical properties of the slurry. The powdery or granular solidmatter is press-molded and subsequently the molded product is fired asit is to give a porous ceramic.

A filter may be used to decrease the liquid component of the slurry,thereby forming a solid matter. Selection of the type of the filter makeit possible to adjust physical properties of the surface of the porousceramic, or surface forms thereof such as smoothness and flatness;adjust the evaporating and scattering property of the slurry when theslurry is solidified; selectively discharge out components whichpermeate the inside of the solid matter; advance crystallization of thehydrated crystalline material from the outside of the solid matter, andthe like. In the case that the solid matter adheres to the filter sothat the matter is not easily striped, it is advisable that a releasingagent, such as silicone, is beforehand applied to the filter.

In the firing step, the solid matter is fired by a known method such asnormal-pressure firing, pressured-gas firing, or microwave-heatingfiring. Thereafter, it is sintered by a sintering method such as hotisostatic press treatment, or glass seal post-treatment. Furthermore,the resultant is subjected to some other heating treatment. At thistime, it is preferable to heat the fired solid matter up to hightemperature so as to cause the porous ceramic to exhibit a sufficientstrength.

In the firing step, the solid matter may be oxidization-fired. In thiscase, a porous ceramic having a high porosity so as to exhibit a lowdensity can be obtained.

In the firing step, the solid matter may be reduction-fired by reducingthe pressure of the gas around the solid matter to remove the gas, orsubstituting the surrounding gas with a nitrogen gas and/or a carbondioxide gas. In this case, a porous ceramic having properties differentfrom those in the case of the oxidization firing can be obtained.

In the firing step, the pressure of the surrounding atmosphere of thesolid matter may be changed while the temperature thereof is keptconstant. Conversely, the temperature of the surrounding atmosphere ofthe solid matter may be changed while the pressure thereof is keptconstant.

In a manner that these matters are combined in the firing step,properties of the resultant porous ceramic can be adjusted.Specifically, for example, at 80 to 100° C., the fluidization anddischarge of the water content can be adjusted by controlling thehumidity, the pressure and temperature of the surrounding of the solidmatter. At 100 to 300° C., the distribution and the size of the porescan be changed by adjusting the temperature and the pressure. At 600° C.or temperatures near it, the sintered state can be decided by selectingoxidization firing or reduction firing. At 900 to 1000° C., the burningtemperature can be abruptly raised by introducing oxygen into reductionatmosphere.

Besides, in an additional step, a surface-covering layer may be formedon the surface of the porous ceramic.

The porous ceramic surface may also be subjected to chemical and/orphysical surface treatment. For example, the porous ceramic is subjectedto physical surface treatment to reform surface properties, such asabsorption, scattering and reflection of light, sounds or electricwaves.

The porous ceramic of the present invention produced as described abovecan be applied to various purposes since the ceramic has a wide surfacearea.

By the use of an aggregate having functionality as the powdery aggregateof the porous ceramic, the ceramic itself can be used as a functionalmaterial.

The porous ceramic formed from a slurry which contains, as powderyaggregates, zeolite, which exhibits the property of absorbing cations,and hydrotalcite, which has anion exchangeability, and also containstrehalose exhibits a performance as an excellent molecule-selectivesieve or adsorbent.

The porous ceramic can be used as a functional material by causing theporous ceramic to carry at least one selected from an electrifiablematerial, a material having an ion exchange group, a material having ahydrophilic functional group, a material having a hydrophobic functionalgroup, a catalyst, an enzyme, an antibody, a protein, a saccharide, alipid, and a nucleic acid; a modified product of any one thereof, acomposite of plural materials selected therefrom; and other ceramicmaterials, and glass.

For example, a photo catalyst wherein the porous ceramic formed from aslurry containing trehalose is caused to carry lamellar titanic acid canalso be applied to organic solar cells since the photo catalyst has avery high reaction efficiency.

The porous ceramic can be used as a filter for capturing biomolecules ofprotein, nucleic acid or the like or letting the biomolecules pass.

For example, a molecular weight marker is fractionated in accordancewith the molecular weight thereof by electrophoresis in a polyacrylamidegel, and the gel is immersed into a tris buffer (SDS, 10% methanol) andtransferred electrically to the porous ceramic made of cordierite. Whenelectric conduction load is then applied thereto for 1 hour or more,molecules which are smaller than myosin (molecular weight: 210 KD)penetrate the membrane of the porous ceramic. On the other hand, myosinremains on the porous ceramic membrane. That is, the porous ceramicfunctions as a molecule filter for causing molecules having a molecularweight of 200 KD or less to pass but capturing molecules having amolecular weight of 200 KD or more. In particular, the porous ceramicformed from a slurry containing glass material as a powdery aggregateand trehalose is useful for separating DNA, RNA or the like fromimpurities since the ceramic exhibits performance for adsorbing nucleicacid into a porous space or releasing nucleic acid therefrom inaccordance with ion strength.

The porous ceramic can be used as a water-cleaning filter, an exhaustgas filter, or a filter for separating gas and liquid from each othersince the porous ceramic allows a liquid or gas of a molecule which issmaller than the pores to pass freely.

The porous ceramic can be used as a space for holding gas or liquidstably.

For example, when the porous ceramic (spray dry, an alumina firedproduct) dried and kept in the air is immersed in water, the ceramicexhibits a performance for releasing gas gently and slowly over 1 houror more. This performance is effective for keeping hydrogen in a fuelbattery, and the like. On the other hand, after the ceramic is immersedin the aqueous solution, the ceramic exhibits a performance for keepinga larger mass than the dry weight in an ambient temperature space over24 hours or more while containing the aqueous solution.

The surface of the porous ceramic can be used as a field formolecule-bonding reaction between biotin and streptavidin, or betweenother compounds.

<Experimental Example>

The porous ceramic of the present invention was produced in accordancewith the following manner.

First, an aqueous supersaturated solution of trehalose (manufactured byHayashibara Shoji, Inc., trade name: Toreha) at 40° C. was prepared, andthereto were added a commercially available dry porcelain material,which is a powdery aggregate, and sodium silicate, which is afluidization agent. The resultant was mixed with an electrically-poweredmixer to form a slurry.

Next, a non-fluidized portion remaining in the bottom of the mixingvessel was removed, and subsequently to the mixture were added {fraction(1/100)} mass % of gua gum (manufactured by Sansho Co., Ltd.) as abinder and antifoamers (manufactured by Nopco Limited, trade name: Nopco8034-L and SN Defoamer 777). The resultant was further mixed to preparean elastic slurry.

Next, to this was added a small amount (about {fraction (1/10000)} mass%) of hyaluronic acid (manufactured by Kibun Chemifa Co., Ltd.)originating from bacteria, and subsequently the resultant was mixed bymeans of the electrically-powered blender. In this way, excessive watercontent was absorbed by hyaluronic acid, to form anelasticity-strengthened slurry.

Next, this elasticity-strengthened slurry was developed onto a nonwovenfabric surface-treated with a releasing agent to produce a plate-formmolded product. Furthermore, thereon was adhered a nonwoven fabricsurface-treated with a releasing agent.

Subsequently, the plate-form molded product sandwiched between thenonwoven fabrics was kept at room temperature for 72 hours to removewater content. In this way, a solid matter was formed.

This solid matter was sintered at 700 to 1350° C. in an oxidizingenvironment, so as to yield a porous ceramic. At this time, excessivewater content, trehalose hydrated crystal and other additives in thesolid matter were evaporated, scattered and burned up so that burned-upportions of the trehalose hydrated crystal, and so on became pores.

FIG. 3 shows a sectional form of the produced porous ceramic.

The porous ceramic produced as described above was a ceramic producedusing the commercially available powdery aggregate having a wideparticle size distribution, but a scattering in pore sizes was verysmall. Moreover, the ceramic exhibited functions of dispersing andattracting solutions and colorant molecules (trypan blue) by a strongcapillary phenomenon. Thus, the ceramic was suitable for variousmolecular sieves.

When the amount of the dissolved trehalose, the temperature of theslurry, and so on were controlled to adjust the amount or the size ofthe trehalose hydrated crystal, porous ceramics having a desiredporosity or pore size were able to be obtained.

When alumina, forsterite, barium titanate, zeolite or the like was usedas a powdery aggregate, a porous ceramic having a very small scatteringin pore sizes was able to be obtained in the same way as describedabove.

Embodiment 2

In the method for producing a porous ceramic according to Embodiment 2of the present invention, first, in a slurry preparing step, a slurrycontaining a powdery aggregate, a burn-out feature material, and amucopolysaccharide was prepared.

Next, in a solid matter forming step, a liquid component is decreasedfrom the slurry prepared in the slurry preparing step, thereby forming asolid matter. The solid matter obtained at this time is a matter whereinthe burn-out feature material is dispersed in the matrix of the powderaggregate.

In a firing step, the solid matter is fired. At this time, the burn-outfeature material is burned up to form a porous ceramic.

The powdery aggregate constitutes the skeleton of the porous ceramic.Examples of the powdery aggregate include pottery, porcelain material,alumina, forsterite,, cordierite, zeolite, hydrotalcite,montomorillonite, sepiolite, zirconia, silicon nitride, silicon carbide,calcium phosphate, hydroxyapatite. Additionally, glass fiber, rock woolalmunosilicate fiber and alumina fiber, which are heat-resistant fibers,are exemplified. The means for adjusting the particle diameter of thepowdery aggregate may be a method using a ball mill. Examples of theshaping method for it include mold-press, cold isostatic press, castmolding, injection molding, extrusion molding, doctor blade methods, andthe like.

The burn-out feature material is dispersed and contained in the solidmatter obtained by removing water content from the slurry and is burnedup by firing, so as to form pores in the porous ceramic. Examples of theburn-out feature material include graphite, wheat flour, starch, phenolresin, polymethyl methacrylate resin, polyethylene resin, andpolyethylene terephthalate resin, and the like. Any one of these or twoor more thereof may be incorporated as the burn-out feature material(s)in the slurry.

A mucopolysaccharide is the generic name of any polysaccharide whichcontains an amino sugar and uronic acid. A mucopolysaccharide absorbswater content in the slurry to remove the water content, and promotescrystallization of soluble components dissolved in the slurry. Byincorporating a mucopolysaccharide in the slurry, a porous ceramichaving a very small scattering in pore sizes can be obtained in the sameway as in the case of Embodiment 1. The mechanism thereof is unclear.Examples of the mucopolysaccharide include hyaluronic acid, chondroitinsulfate, heparin, and keratan sulfate. Furthermore, pectin, carrageenan,or the like may be used as an uronic acid analogue. Among these,hyaluronic acid, which is excellent in water absorbing performance, ispreferable.

FIG. 2 is a view showing the structural formula of hyaluronic acid.

Hyaluronic acid is a macromolecular mucopolysaccharide having, as aminimum recurring unit, a disaccharide of glucuronic acid andN-acetyl-D-glucosamine. Hyaluronic acid has a property of attracting agreat amount of water and attracts water content in a volume 6000 timesthe weight thereof. Therefore, only by incorporating a very small volumeof hyaluronic acid in the slurry, hyaluronic acid absorbs most all ofwater content in the slurry in a liquid form, so that the slurry isstabilized. That is, by the fixation of excessive water content, thefluidity of the slurry which is close to the critical point of sol-gelis reduced at a stretch to result in gelatinization.

In the case that the mucopolysaccharide is, for example, hyaluronicacid, aggrecan, which is bonded to hyaluronic acid to form a giantcomplex, may be incorporated in the slurry. Aggrecan is keratansulfate/chondroitin sulfate proteoglycan having a molecular weight ofabout 2500 KD.

A binder for bonding powders of the powdery aggregate to each other maybe incorporated in the slurry. In the case that the volume of theburn-out feature component distributed between the powdery aggregatepowders is large, the bonds between the powdery aggregate powders areliable to become brittle. However, the binder causes pore spaces betweenthe powdery aggregate powders to be kept while the binder makes strongmutual bonds between them. Examples of the binder include sodiumsilicate, colloidal silica, alumina sol, lithium silicate, glass frit,gua gum (natural rubber, or synthetic rubber), methylcellulose,carboxymethylcellulose, hydroxymethylcellulose, hydroxyethylcellulose,polyvinyl alcohol, polyethylene, phenol resin, polyacrylate, and sodiumpolyacrylate. Any one of these or two or more thereof may beincorporated as the binder(s) in slurry.

At least one selected from the following may be incorporated in theslurry: a fluidizing agent such as sodium silicate, a dispersing agentsuch as polycarboxylic acid, a slurry hardening agent of amine type orthe like, an organic monomer and a crosslinking agent for the monomer, aplasticizer, an antifoamer, and a surfactant such as sugar ester orpolygly ester. When such an additive is incorporated in the slurry,properties of the slurry or the solid matter change. Properties of theresultant pore ceramic, such as the strength thereof and the formingmanner of the pores, are affected and changed accordingly. For example,when a surfactant is incorporated in the slurry, the state that water isblended with the powdery aggregate is made more homogeneous so that thehomogeneity of the resultant pore ceramic becomes high. Consequently, byincorporating a given additive in the slurry dependently on purpose, adesired porous ceramic can be obtained.

Besides, an appropriate amount of one or more of the following organiccompounds may be incorporated in the slurry: other sugars and aminoacids, such as glucose, maltose, isomerized sugars, sucrose, honey,maple sugar, palatinose, sorbitol, xylitol, lactitol, maltitol,dihydrochalcone, stevioside, α-glycosylstevioside, rakanka sweet tastematerial, glycyrrhizin, L-aspartyl-L-phenylalanine methyl ester,sucralose, Acesulfame K, saccharin, glycine, alanine, dextrin, starch,and lactose.

In the solid matter forming step, water content is removed from theslurry by such means as hot wind drying, freeze-drying, super criticaldrying, or spray drying. By lowering the humidity of the surroundingatmosphere of the slurry, the evaporation of the water content from theslurry may be promoted. Furthermore, by lowering the pressure of thesurrounding atmosphere of the slurry, the evaporation of the watercontent from the slurry may be promoted.

In the solid matter forming step, a powdery or granular solid matter maybe formed by spray-drying the slurry in a spray drying machine (spraydryer). Specifically, the slurry is blown out from nozzles to a dryingchamber in a heated state, so as to prepare liquid droplets in a fineparticle form, and the liquid droplets are brought into contact with hotwind and dried, thereby yielding a powdery or granular solid matter. Thespray drying machine is classified into a disc atomizer, a two-fluidnozzle, a four-fluid nozzle, or other types, dependently on differencein its pulverizing device. An appropriate type is selected in accordancewith physical properties of the slurry. The powdery or granular solidmatter is press-molded and subsequently the molded product is fired asit is to give a porous ceramic.

A filter may be used to decrease the liquid component of the slurry,thereby forming a solid matter. Selection of the type of the filter makeit possible to adjust physical properties of the surface of the porousceramic, or surface forms thereof such as smoothness and flatness;adjust the evaporating and scattering property of the slurry when theslurry is solidified; selectively discharge out components whichpermeate the inside of the solid matter; advance crystallization of thehydrated crystalline material from the outside of the solid matter, andthe like. In the case that the solid matter adheres to the filter sothat the matter is not easily striped, it is advisable that a releasingagent, such as silicone, is beforehand applied to the filter.

In the firing step, the solid matter is fired by a known method such asnormal-pressure firing, pressured-gas firing, or microwave-heatingfiring. Thereafter, it is sintered by a sintering method such as hotisostatic press treatment, or glass seal post-treatment. Furthermore,the resultant is subjected to some other heating treatment. At thistime, it is preferable to heat the fired solid matter up to hightemperature so as to cause the porous ceramic to exhibit a sufficientstrength.

In the firing step, the solid matter may be oxidization-fired. In thiscase, a porous ceramic having a high porosity so as to exhibit a lowdensity can be obtained.

In the firing step, the solid matter may be reduction-fired by reducingthe pressure of the gas around the solid matter to remove the gas, orsubstituting the surrounding gas with a nitrogen gas and/or a carbondioxide gas. In this case, a porous ceramic having properties differentfrom those in the case of the oxidization firing can be obtained.

In the firing step, the pressure of the surrounding atmosphere of thesolid matter may be changed while the temperature thereof is keptconstant. Conversely, the temperature of the surrounding atmosphere ofthe solid matter may be changed while the pressure thereof is keptconstant.

In a manner that these matters are combined in the firing step,properties of the resultant porous ceramic can be adjusted.Specifically, for example, at 80 to 100° C., the fluidization anddischarge of the water content can be adjusted by controlling thehumidity, the pressure and temperature of the surrounding of the solidmatter. At 100 to 300° C., the distribution and the size of the porescan be changed by adjusting the temperature and the pressure. At 600° C.or temperatures near it, the sintered state can be decided by selectingoxidization firing or reduction firing. At 900 to 1000° C., the burningtemperature can be abruptly raised by introducing oxygen into reductionatmosphere.

Besides, in an additional step, a surface-covering layer may be formedon the surface of the porous ceramic.

The porous ceramic surface may also be subjected to chemical and/orphysical surface treatment. For example, the porous ceramic is subjectedto physical surface treatment to reform surface properties, such asabsorption, scattering and reflection of light, sounds or electricwaves.

The porous ceramic of the present invention produced as described abovecan be applied to various purposes in the same way as in Embodiment 1since the ceramic has a wide surface area.

INDUSTRIAL APPLICABILITY

The porous ceramic of the present invention and the method for producingthe same contribute largely to technique improvement in various fieldsof nanotechnology, biotechnology, medical science and medical treatment,and environment preservation.

1. A method for producing a porous ceramic, comprising: the slurrypreparing step of preparing a slurry containing a powdery aggregate anda hydrated crystalline material having a burn-out feature, the solidmatter forming step of decreasing a liquid component from the slurryprepared in the slurry preparing step, thereby forming a solid matter,and the firing step of firing the solid matter formed in the solidmatter forming step to burn up the hydrated crystalline material,thereby forming the porous ceramic.
 2. The method for producing theporous ceramic of claim 1, wherein the hydrated crystalline material istrehalose.
 3. The method for producing the porous ceramic of claim 2,wherein in the slurry preparing step, a given thermal hysteresis isgiven to the slurry so as to set the size of crystal of the trehalose toa given size.
 4. The method for producing the porous ceramic of claim 1,wherein in the slurry preparing step, a mucopolysaccharide isincorporated in the slurry.
 5. The method for producing the porousceramic of claim 4, wherein the mucopolysaccharide is hyaluronic acid.6. The method for producing the porous ceramic of claim 1, wherein inthe slurry preparing step, a binder for binding powders of the powderyaggregate to each other is incorporated in the slurry.
 7. The method forproducing the porous ceramic of claim 6, wherein the binder is at leastone selected from sodium silicate, colloidal silica, alumina sol,lithium silicate, glass frit, natural rubber, synthetic rubber,methylcellulose, carboxymethylcellulose, hydroxymethylcellulose,hydroxyethylcellulose, polyvinyl alcohol, polyethylene, phenol resin,polyacrylate, and sodium polyacrylate.
 8. The method for producing theporous ceramic of claim 1, wherein in the slurry preparing step, atleast one selected from the following is incorporated in the slurry: afluidizing agent, a dispersing agent, a slurry hardening agent, anorganic monomer and a crosslinking agent for the monomer, a plasticizer,an antifoamer, and a surfactant.
 9. The method for producing the porousceramic of claim 1, wherein in the slurry preparing step, a burn-outfeature material which is different from the hydrated crystallinematerial is incorporated in the slurry.
 10. The method for producing theporous ceramic of claim 1, wherein in the slurry preparing step, theslurry is cooled from the outside.
 11. The method for producing theporous ceramic of claim 1, wherein in the solid matter forming step, thehumidity of the surrounding atmosphere of the slurry is lowered.
 12. Themethod for producing the porous ceramic of claim 1, wherein in the solidmatter forming step, the pressure of the surrounding atmosphere of theslurry is lowered.
 13. The method for producing the porous ceramic ofclaim 1, wherein in the solid matter forming step, the slurry isspray-dried, thereby forming the solid matter in a powder or granularform.
 14. The method for producing the porous ceramic of claim 1,wherein in the solid matter forming step, a filter is used to decreasethe liquid component of the slurry, thereby forming the solid matter.15. The method for producing the porous ceramic of claim 1, wherein inthe firing step, the solid matter is oxidization-fired.
 16. The methodfor producing the porous ceramic of claim 1, wherein in the firing step,the solid matter is reduction-fired.
 17. The method for producing theporous ceramic of claim 1, wherein in the firing step, the pressure ofthe surrounding atmosphere of the solid matter is changed while thetemperature thereof is kept constant.
 18. The method for producing theporous ceramic of claim 1, wherein in the firing step, the temperatureof the surrounding atmosphere of the solid matter is changed while thepressure thereof is kept constant.
 19. A porous ceramic produced byfiring a solid matter formed by decreasing a liquid component from aslurry containing a powdery aggregate and a hydrated crystallinematerial having a burn-out feature so as to burn up the hydratedcrystalline material.
 20. The porous ceramic of claim 19, wherein thehydrated crystalline material is trehalose.
 21. The porous ceramic ofclaim 19, which carries at least one selected from an electrifiablematerial, a material having an ion exchange group, a material having ahydrophilic functional group, a material having a hydrophobic functionalgroup, a catalyst, an enzyme, a protein, a saccharide, a lipid, and anucleic acid; a modified product of any one thereof, a composite ofplural materials selected therefrom; and other ceramic materials, andglass.
 22. The porous ceramic of claim 19, wherein a surface-coveringlayer is formed.
 23. The porous ceramic of claim 19, wherein the surfacethereof is subjected to chemical and/or physical surface treatment. 24.The porous ceramic of claim 19, wherein the pore density of an outerportion is higher than that of an inner portion.
 25. A method forproducing a porous ceramic, comprising: the slurry preparing step ofpreparing a slurry containing a powdery aggregate, a burn-out featurematerial, and a mucopolysaccharide, the solid matter forming step ofdecreasing a liquid component from the slurry prepared in the slurrypreparing step, thereby forming a solid matter, and the firing step offiring the solid matter formed in the solid matter forming step to burnup the burn-out feature material, thereby forming the porous ceramic.26. The method for producing the porous ceramic of claim 25, wherein themucopolysaccharide is hyaluronic acid.
 27. The method for producing theporous ceramic of claim 26, wherein in the slurry preparing step,aggrecan is incorporated in the slurry.
 28. The method for producing theporous ceramic of claim 25, wherein in the slurry preparing step, abinder for binding powders of the powdery aggregate to each other isincorporated in the slurry.
 29. The method for producing the porousceramic of claim 28, wherein the binder is at least one selected fromsodium silicate, colloidal silica, alumina sol, lithium silicate, glassfrit, natural rubber, synthetic rubber, methylcellulose,carboxymethylcellulose, hydroxymethylcellulose, hydroxyethylcellulose,polyvinyl alcohol, polyethylene, phenol resin, polyacrylate, and sodiumpolyacrylate.
 30. The method for producing the porous ceramic of claim25, wherein in the slurry preparing step, at least one selected from thefollowing is incorporated in the slurry: a fluidizing agent, adispersing agent, a slurry hardening agent, an organic monomer and acrosslinking agent for the monomer, a plasticizer, an antifoamer, and asurfactant.
 31. The method for producing the porous ceramic of claim 25,wherein in the solid matter forming step, the humidity of thesurrounding atmosphere of the slurry is lowered.
 32. The method forproducing the porous ceramic of claim 25, wherein in the solid matterforming step, the pressure of the surrounding atmosphere of the slurryis lowered.
 33. The method for producing the porous ceramic of claim 25,wherein in the solid matter forming step, the slurry is spray-dried,thereby forming the solid matter in a powder or granular form.
 34. Themethod for producing the porous ceramic of claim 25, wherein in thesolid matter forming step, a filter is used to decrease the liquidcomponent of the slurry, thereby forming the solid matter.
 35. Themethod for producing the porous ceramic of claim 25, wherein in thefiring step, the solid matter is oxidization-fired.
 36. The method forproducing the porous ceramic of claim 25, wherein in the firing step,the solid matter is reduction-fired.
 37. The method for producing theporous ceramic of claim 25, wherein in the firing step, the pressure ofthe surrounding atmosphere of the solid matter is changed while thetemperature thereof is kept constant.
 38. The method for producing theporous ceramic of claim 25, wherein in the firing step, the temperatureof the surrounding atmosphere of the solid matter is changed while thepressure thereof is kept constant.
 39. A porous ceramic produced byfiring a solid matter formed by decreasing a liquid component from aslurry containing a powdery aggregate, a burn-out feature material, anda mucopolysaccharide so as to burn up the hydrated crystalline material.40. The porous ceramic of claim 39, wherein the mucopolysaccharide ishyaluronic acid.
 41. The porous ceramic of claim 39, which carries atleast one selected from an electrifiable material, a material having anion exchange group, a material having a hydrophilic functional group, amaterial having a hydrophobic functional group, a catalyst, an enzyme, aprotein, a saccharide, a lipid, and a nucleic acid; a modified productof any one thereof, a composite of plural materials selected therefrom;and other ceramic materials, and glass.
 42. The porous ceramic of claim39, wherein a surface-covering layer is formed.
 43. The porous ceramicof claim 39 wherein the surface thereof is subjected to chemical and/orphysical surface treatment.